Cathode-ray tube tester



Dec. 5, 1950 R. s. coATE CATHODE-RAY TUBE TESTER Filed sept. so', 1949 :inventor Gttorneg atentecl Dec. 5,

CATHODE-RAY TUBE TESTER Ralph S. Coate, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1949, Serial No. 118,911

7 Claims.

This invention relates to cathode ray tube testing apparatus, and particularly to such an apparatus utilizing only alternating voltages.

The widespread use of cathode ray tubes in Oscilloscopes, television cameras and receivers, navigational aids, and the like, has developed a need for a simple system of testing such tubes. Various complicated devices have been proposed for this purpose, but since such devices usually involve a relatively large number of control dials, as well as various meters and indicators, all of which complicate the problem of obtaining a rapid check on the operating characteristics of a tube, they are inconvenient, to say the least, for making rapid tests.

Since cathode ray tubes generally are operated with unidirectional voltages of suitable polarity and magnitude, it is customary to use unidirectional voltages in cathode ray tube testing apparatus. However, most power supply systems provide alternating voltages to the consumer, so that various rectiiying circuits are required for producing unidirectional voltages in cathode ray tube testing apparatus, and al1 such circuits add to the expense and complexity of the testing equipment.

I have found that the most commonly encountered faults in cathode ray tubes can be detected with alternating test voltages, and that a simple and efficient testing apparatus can be constructed which does not require the use of rectiers, amplifiers, or similar expensive components, and one which involves a minimum number of controls and indicators.

It is, accordingly, a general object of the present invention to provide a simplified apparatus for testing cathode ray tubes.

Another object of the invention is to provide an alternating voltage cathode ray tube tester.

A further object of the invention is to provide a tester of the foregoing type adapted to test either electrostatic deflection type or magnetic deflection type cathode ray tubes.

In accordance with the invention, the foregoing and other related objects and advantages are attained in an apparatus wherein means are provided for obtaining alternating voltages of suitable magnitude for operating a cathode ray tube, together with phase shifting networks arranged to overcome certain of the diiculties which otherwise would be encountered in a cathode ray tube testing system utilizing only alternating voltages.

A more complete understanding of the invention can be had from the following description of an illustrative embodiment thereof, When considered in connection with the accompanying drawing, the single gure of which is a schematic diagram of a cathode ray tube testing apparatus embodying the principles of my invention, together with typical cathode ray tubes which can be tested with apparatus of the type contemplated herein.

Referring to the drawing, there is shown a socalled electrostatic deflection type cathode ray tube lo having an electron-beam-iorming gun comprising a cathode l2, adapted to be heated by a lament lli to serve as a source of electrons, a control grid i6 to regulate the electron beam current, a iirst or focusing anode i8 which cooperates with the electrodes l2, IS to form an electron lens for focusing the cathode ray beam, a high voltage anode 2li which serves to accelerate the electrons in the beam, two pairs of orthogonally related beam deflecting plates 2U, 22, and a uorescent screen 25 upon which the electron beam impinges to produce a desired image. In some cases, the tube li? may contain a further anode comprising a conductive coating 25 adjacent the screen 25. This additional anode often is connected internally to the high voltage anode 24, and, if not so connected, can be connected externally for testing purposes, as shown.

Also shown in the drawing is a cathode ray tube 3@ of the so-called magnetic deflection type containing a cathode i2, a lament iii, a, control grid IS, a second or screen grid 38, and a high voltage anode 25. As is well known, the purpose of the screen grid 33 is to screen the region between the cathode I2 and the anode 25 in order to reduce the value of negative control grid voltage necessary to cut off the beam current. Focusing and deflection coils (not shown) usually are placed around the neck of the tube I9 in the region between the screen grid 38 and the anode 25 to focus and deect the electron beam in a manner well known in the art.

It will be understood that the tubes lil, 39 are merely illustrative of the two major types of cathode ray tubes, and that a testing apparatus of the type to be described can be modified as necessary to accommodate any particular tube of either type.

The testing apparatus shown in the drawing is adapted to provide suitable voltages for operating either the electrostatic tube I5 or the electromagnetic tube 30 in order that certain tests can be made, as will be described, and comprises a transformer 5i] having a primary Winding 52, connected through a switch 54 to a pair of input terminals 5S, and three secondary windings 5S, SQ, t2. In the drawing, direct connections are shown from various points on the secondary windings 58, tti, 62, as well as from other points in the testing circuit, to selected electrodes in the tubes iti, 3%, it being understood that sets of adaptor plugs and sockets can be provided in order to make these connections to diierent types oi cathode ray tubes. For example, in a typical apparatus embodying my invention, it was found that 7 sets of adaptor plugs were adequate to' allow testing of 32 diierent types of oscilloscope and television tubes. For the sake of simplicity, the testing apparatus is shown connected to both of the tubes i6, 3Q, it being understood that only a single tube would be connected to the apparatus under operating conditions, and that adaptor plugs and sockets would be 'utilized 'as already mentioned. f

One of the secondary windings 58 on the transformer 50 is connected to the laments I4 in the tubes i9, 30, and one end of the lament winding 58 is connected to an end terminal l@ of the adjacent winding Sil through a glow tube indicator i2 and a resistor le, A potentiometer type resistor 'l5 is connected between the end terminal l@ and a second terminal E8 of the secondary winding 6G, with the potentiometer arm ita being connected to the control grids IE of the tubes lil, 35, and the second terminal i8 being connected to the tube cathodes l 2.

Since the indicator i2 is connected in circuit between the larnent winding 58 and the terminal i8 (which is connected to the tube cathodes I2), if the tube being tested has a short between the lament and the cathode, this fact will be indicated by the indicator '52. The purpose of the potentiometer it will be referred to hereinafter.

A pair of resistors 88, 82 are connected to form a voltage divider between the terminal 'i8 and a third terminal 8, and a connection is made from the junction 8i of the resistors 89, 82 to the screen grid 3% oi the tube 39 through a resistor S5, while a connection is made from the third .5"

terminal 84 to the rst anode I8 of the tube It through a resistor 81.

The end terminal B on the transformer winding Sii is connected through a resistor 88 to a iixed Contact 9S of a normally closed switch 92, as well as to one terminal Se of the secondary winding 52. The other terminal Q of the winding 52 is connected through a resistor 98 to the movable contact 93 oi the switch 92, and is also connected through a glow tube indicator it? to ground and to the high voltage anodes 2li of the tubes li), Se.

It will be noted that the control grid I5 and the anode 24 or" the tube i9 are connected to the transformer winding BS (at the potentiometer arm ld, and at the terminal S5, respectively) on opposite sides of the cathode connection (terminal '58), so that the voltage from the control grid l@ to the cathode l2 will be 180 out of phase with the anode-to-cathode Voltage of the tube l il. Similarly, the cathode i2 of the tube 3Q is connected to the transformer winding 6B (at terminal r8) between the control grid connection (potentiometer arm ltd) and the anode connection (terminal St), so that the voltage from the control grid l to the cathode l2 will be 180 out of phase with the anode-to-cathode voltage o f the tube 3S. These connections will provide negative bias voltages for the control grids i6 in the tubes it, at times when the other electrode voltages are positive with respect to the cathode, thereby providing suitable biasing for proper tube operation. The magnitude of this bias voltage can be controlled by adjustment of the potentiometer arm 16a.

As long as the switch 92 is closed, the high voltage anode 24 of the tube being tested will receive voltage from the terminal 86 of the transformer winding 6@ through the switch 92 and the indicator l, since the switch 92 will short out the voltage across the winding 62. The Voltage between the terminals 86, 18 of the winding te should be approximately equal to the lowest normal operating voltage of any tube type for which the tester is designed, so that any tube tested will produce a luminous spot on the tube screen. This, alone, will indicate that the tube is operative. If the tube being tested appears to glow internally, it usually indicates insufficient degas- Sing or gas leakage. By varying the setting of the potentiometer arm hid, the grid bias can be changed, and this should produce a change in the intensity oi the luminous spot. If no change in spot intensity results, either a control-grid to cathode short or, possibly, a faulty grid connection, is indicated for the tube being tested.

When the switch 52 is opened, additional voltage (across the winding 62) will be applied between the high voltage anode 2l! and the cathode l2 of the tube being tested. This additional voltage is made large enough so that arcing to the high voltage anode will occur if the tube has any defects likely to cause such arcing during normal operation. The indicator Ill@ is a two element tube of the type which will glow on one side in response to a unidirectional voltage, and will glow on both sides in response to a bi-directional voltage. If the high voltage anode current is normal (i. e. unidirectional), only one side of the glow tube l will be ignited. However, when arc-over occurs, current will flow in both directions through theA indicator it, causing both sides of the tube H10 to glow, thereby indicating arcing or high resistance leakage in the tube being tested.

The arrangement shown for applying additional anode voltage to the` tube being tested is preferable to a switching arrangement wherein the anode connectionI is shifted between two high potential contacts, because with the arrangement shown, only one switch Contact needs to be insulated for high voltage, whereas with a two contact system of the type mentioned, both contacts of the switch would require high voltage insulation.

The resistor 7,5 in the control grid circuit is provided to protect the tube-grid by limitingY the grid current during positive half cycles of grid voltage, while the resistors 14, 85, 8l, 88, and 3 8,y are providedI to limit the current drawn through the transformer windings by the tube being tested in order to protect the transformer against heavy. short circuit currents, as well as to protect personnel inadvertentlycoming in contact with any of the tester output terminals.

The voltageA divider resistors 8l), S2 .determine the magnitude of the screen grid voltage for mag-4, netic deiiection type tubes, such as thev tube 30, and could as well be replaced by an additional tap on the secondary winding '69.

Preferably, the high voltage anode connection is grounded, as shown, in order to protect personnel testing tubes of the type having an eX- ternal anode capy connector which might accidentally betouchedby the operator.

The testing apparatus also includes a second transformer |02, having a primary winding IM which is connected to the inputI terminals 56 through a resistance-capacitance phase shifting network Iii. |The network |06 is arranged in conventional manner to produce substantially a 90 phase displacement between the voltage across the primary winding It and the voltage at the input terminals 56. A first secondary winding IS of the transformer I2 is connected to the vertical deflection plates 2i) in the tube I, while a` second secondary winding IED of the transformer |52 is connected to a coil H2 which is intended to serve as a beam deflection coil for the tube 3b.

The purpose of the phase shift network IilI will be clear from a consideration of the effect of applying in-phase alternating voltages to the electron gun and to the beam deflection system of the tube being tested. With in-phase voltages, maximum beam deection would occur at the time that the cathode-anode voltage is at a maximum, so that the beam would be deiiected off of the screen at the time maximum anode voltage occurs. This, of course, would be objectionable because little if any image would be produced on the tube screen. Consequently, the phase shift network |06 is provided so that the deflection voltage will be displaced in phase with respect to the anode-cathode voltage. Under these conditions, the alternating deflection voltage will sweep the beam repeatedly across the tube screen, and since the deflection voltage will be zero at the time when the anode-cathode voltage is maximum, the tube screen will be illuminated by a beam trace having maximum intensity at the center. By making the peak-to-peak value of the deflection voltage large enough to deflect the beam well beyond both edges of the screen, a trace of substantially uniform intensity can be obtained on the tube screen and this trace will serve to bring out any defects in the screen along the scanning path.

A capacitor I preferably is connected in parallel with the winding IM, and the capacitor and winding tuned approximately to parallel resonance at the frequency of the alternating voltage source in order to limit the current drawn through the phase shift network. This serves to increase the useful life of the phase shift network elements.

In the case of magnetic deflection type tubes, such as the tube 3G, the trace on the tube screen can be rotated to allow examination of all parts of the screen simply by relative rotation of the deflection yoke and the tube being tested. However, since the deflection plates are fixed in position in the electrostatic tube It, some other means is required for shifting the beam.

To this end, a variable phase shift network II4 is connected between the input terminals 56 and the horizontal deflection plates 22 in the tube I0, preferably through an isolation transformer IIE. By simultaneously varying the setting of a pair of potentiometer-type resistors I IE, I I3 in the phase shift network Hd, the phase angle between the voltages on the deflection plates 20, 22 can be varied approximately from Zero to plus or minus d5 degrees, thereby changing the deflection pattern of the cathode ray beam from a straight line (at zero phase angle) into an elliptical pattern which gradually broadens out as the phase angle between the deection voltages increases in one direction or the other. However, it should be noted that only one side of the ellipse will appear as a trace on the screen, since the anode-to-cathode voltage will be negative during half of each cycle, thereby eliminating one half of the elliptical trace. As the voltage on the deflection plates 22 is advanced or retarded in phase by adjustment of the phase shift network I I4, the elliptical deflection patternwill broaden out, and the visible portion thereof will move toward one side or the other of the screen. If the peak-to-peak values of deflection voltage are made sufficiently large, the trace on the screen will appear as a nearly straight line which can be moved back and forth across the screen to allow examination for defects.

In order that the voltages on the deflection plates will be suitably balanced with respect t0 anode voltage, resistors |20, |22, |24, and |25 are connected between the deiiection plates 20, 22 and ground.

It can be seen that the apparatus shown will provide an indication (a) of tube operativeness (by screen illumination); (b) of insuiiicient degassing or gas leakage (by internal glow) (c) of heater to cathode shorts (by the indicator l2) (d) of control grid cathode shorts, or open control grid connections (by effect of varying the potentiometer 76); (e) of high voltage anode arcs (by the indicator Ilm); (f) of screen defects (by moving the trace on the tube screen). Since the faults listed are among those most often encountered with cathode ray tubes, it is evident that a simple and eiicient testing apparatus is provided which will furnish a rapid check of such tubes.

Since many changes could be made in the apparatus shown and described, all within the scope and spirit; of the invention, the foregoing is to be construed as illustrative, and not in a limiting sense.

What is claimed is:

1. An alternating current apparatus for testing cathode ray tubes of the type including an anode and a cathode for generating an electron beam, an electrode for controlling the intensity of said beam, and means for deflecting said beam, said apparatus comprising a pair of input terminals adapted to be connected to a source of alternating voltage, a transformer having a primary winding connected to said input terminals and a secondary winding having output terminals, circuit means to connect one of said output terminals to said cathode and another of said output terminals to said anode to apply between said anode and cathode an alternating voltage derived from said source, a phase shifting network connected to said input terminals, and means to connect said network to said beam deiiecting means to furnish to said de- :Iiecting means an alternating voltage shifted in phase with respect to the voltage between said anode and said cathode.

2. Apparatus as dened in claim l including a variable phase shifting network, means connecting said variable phase shifting network to said input terminals, and means to connect said variable phase shifting network to said beam deflecting means thereby to provide separate, variably phase-displaced, beam deecting voltages for cathode ray tubes of the type wherein i said beam defiecting means comprises two pairs of orthogonally related beam-deflection plates.

3. Apparatus as dened in claim l including a coil for generating a magnetic beam-deflecting eld and means connecting said coil to said phase shifting network whereby to adapt said assess@ 7 apparatus fortesting cathode ray tubes Yeither of the magnetic deflection type or of the electr-ostatic deection ty-pe.

4. Apparatus as defined in claim l wherein said one output terminal is disposed between said another output terminal and a third output terminal on said secondary winding, a potentiometer-type resistor connected between said one output terminal and said third output terminal and having a movable tap, and means to connect said tap to said beam control electrode in said cathode ray tube whereby to provide outof-.phase voltages between said anode and cathode and between said control grid and cathode.

5. Apparatus as dened in claim l wherein said circuit means to connect said another output terminal to said anode includes a second secondary winding on said transformer, a normally closed switch connected across said second secondary winding to. normally connect said anode directly to said another output terminal whereby to add the voltage across said second secondary winding to the normal voltage between said anode and cathode by opening said switch, and means in circuit with said second secondary winding for indicating alternating current flow to said anode.

6. In an apparatus for testing a cathode ray tube comprising an anode and a cathode for generating an electron beam and means for deflecting said beam, in combination, a pair of input terminals adapted to be connected to. a source ofv alternating voltage, output yterminals adapted to be connected to said anode and said cathode, a circuit including voltage step-.up means connected between said input and output terminals to develop from alternating voltage at said input terminals an alternating voltage of increased magnitude across said output terminals, and a circuit including phase shifting means for connecting said beam deecting meansy to said input terminals to transfer alter-. nailing voltage from said` input. terminals to said beam deflecting meansv in phase shifted relation` with respect, to said voltage across said 45 outputY terminals.

7. Apparatus for testing a cathode ray tube having a cathode, a control grid, an anode, beam delecting means, and a cathode-heater filament, said apparat-us comprising a transformer having a primary winding, a first secondary winding on said transformer having a plurality of output terminals intermediate the ends thereof, -a potentiometer-type resistor connected between one end of said rst winding and a rst terminal adjacent thereto and having a movable arm, means to connect said control grid and said --cathode to said movable arm and to said first terminal, respectively, a second secondary winding on said transformer connected in series with said rst winding at the other end of said first winding, a normally closed switch connected across said second winding, means to connect said windings and said indicator in series with said anode, said indicator being adapted lto indicate alternating current flow to said anode, a thi-rd secondary winding on said transformer, means to connect said lament across said third winding, an indicator connected to indicate current flow between said iirst and third windings, a pair of input terminals adapted to be connected to a source of alternating voltage, connections from said primary winding to said input terminals, .a second transformer having a primary winding and a secondary winding, a phaseshiftin r circuit connecting said second transformer primary Winding to said input terminals, and means to connect said secondI transformer secondary winding to said beam deecting means.

RALPH S. COATE.

REFERENCES CITED UNITED STATES PATENTS Name Date Crost June 29, 19.4'8

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